It is known that, prior to an aircraft take-off, the pilot determines a value VR of the speed of the aircraft, called the rotation speed, at which the take-off rotation must start, at the end of the acceleration phase during which said aircraft runs on the take-off runway whilst accelerating. This rotation speed VR is a calculation of optimization of the take-off of the aircraft taking into account both the characteristics of said aircraft (mass, thrust, configuration of slats and flaps, etc) and the characteristics of the take-off runway (length, altitude, state, temperature, etc). Furthermore, it must be within a range of speed values imposed by regulations.
Moreover, it is known that certain aircraft comprise a horizontal stabilizer with adjustable inclination. Such a trimmable horizontal stabilizer is, in the art, described by one or other of the abbreviations PHR (for Plan Horizontal Réglable) or THS (for Trimmable Horizontal Stabilizer). Just like a fixed horizontal stabilizer, a trimmable horizontal stabilizer is provided with elevators forming the trailing edge of said trimmable horizontal stabilizer.
A trimmable horizontal stabilizer can be deflected in the pitch-up or pitch-down direction and it is used in certain flight phases. For example, during the take-off of the aircraft and prior to rotation, it is usual to deflect said trimmable horizontal stabilizer, by the action of a pilot or of an automatic system, by a deflection angle of predetermined value. The theoretical optimum value of the deflection angle of the trimmable horizontal stabilizer depends on several parameters of the aircraft, such as the longitudinal position of the center of gravity, the total mass on take-off, the configuration of the leading edge slats and of the trailing edge flaps, the thrust, the speed at which the rotation takes place, etc.
The real value of the deflection angle is important because it conditions the behavior of the aircraft during the rotation. If the real value of that deflection angle is too pitch-up, this can be followed by a spontaneous take-off without the intervention of the pilot with a possible touch-down of the tail or, on the contrary, if it is too pitch-down, by a laborious take-off having a negative effect on the aircraft performance.
As a general rule, on take-off, the value of the deflection angle of the trimmable horizontal stabilizer corresponds to a pitch-up moment, which is particularly the case when the center of gravity of the aircraft occupies a forward longitudinal position towards the nose of the aircraft: in fact, in this case, the aircraft is difficult to turn at the moment of the rotation and the trimmable horizontal stabilizer must create a high pitch-up moment. However, when the center of gravity of the aircraft is in a rearward longitudinal position, the aircraft has a tendency to turn very easily and the trimmable horizontal stabilizer must create only a weak pitch moment, which can be pitch-up or even pitch-down.
As recalled above, the theoretical optimum value of the deflection angle, at take-off, of the trimmable horizontal stabilizer depends on numerous parameters. Thus, for an accurate adjustment of the inclination of said trimmable horizontal stabilizer, it is necessary to take account of the totality, or at least of a large number, of these parameters, which results in complicated rules.